The present invention relates to an electrochemical method and apparatus for treating plating plant wastewater to remove heavy metal and other contaminants, including toxic organic materials, chelating agents and cyanide which may be present in the wastewater.
During many commercial plating operations, wastewater is generated which contains heavy metal contaminants such as copper, zinc, nickel, lead and cadmium, and other toxic materials, as well as chelating agents. Such wastewater particularly results from electroless and electroplating processes and from metal finishing processes. Because of their toxicity, removal of these metals is desirable. Removal of chelating agents is also desirable because they can disrupt subsequent removal by chemical precipitation operations. Thus, simultaneous removal of heavy metals and destruction of chelating agents is desirable. Additionally, certain plating processes may produce cyanide as a contaminant. Accordingly, it is also desirable to remove this contaminant in plating wastewater treatment processes.
In any electrochemical cell for treating wastewater of the present kind, there are certain very stringent requirements imposed upon the cathode and anode materials. In particular, it is required that the cathode especially exhibit a large reaction surface area. Additionally, the porosity of the cathode should be large (in excess of 85%) to minimize flow restriction. The cathode material should also be able to withstand high levels of metal loading before exhaustion, so that excessively frequency replacement of the cathode material is not required. Moreover, since at least some electrode replacement is necessary during operation of such treatment systems because of electrodeposition of copper or other metal, the cell should be constructed to allow easy disassembly and replacement of the electrodes.
U.S. Pat. No. 3,859,195, issued Jan. 7, 1975 to John M. Williams appears to describe an apparatus for electrochemical processing of ionically-conductive solutions. This patent discusses various previous forms that electrodes have taken, listing, in particular, porous carbon electrodes, carbon block electrodes, electrodes comprising packed beds of carbon particles, fluidized beds of carbon particles, and metallic extended surface electrode structures in the form of loose stainless steel wool pads, citing, in particular, U.S. Pat. No. 2,588,450. The Williams patent appears to disclose the use of woven metallic mesh electrodes in a flow-through mode. However, an important aspect of the device in the Williams patent is the inability of the cathode to be removed from the electrolytic cell and its inability to prevent electrodes from shorting, since it is in a spiral configuration. As is seen below, such construction is inconsistent with the objectives of the instant invention; in particular, in the present invention, a flow-through cathode of different material and design is replaceable, thereby allowing its repeated use in wastewater treatment cycles.
In U.S. Pat. Nos. 4,046,663 and 4,046,644, there are apparently disclosed electrodes comprising carbon fibers or carbon fibers coated with a metallic coating. The electrodes shown there are loose carbon fiber brushes which do not have sufficient electrically conducting contacts for efficient current collection. Alternatively, woven carbon fiber electrodes would be too dense for effective mass transfer of pollutants to the electrode surface. In contrast, the instant invention employs a highly porous cathode, made of non-woven carbon fibers, with excellent current collection designed specifically for metal recovery operations.
With respect to other known electrodes for electrochemical reactors, these electrodes have included such structur as fluidized beds of metal spheres, metal-coated glass spheres, carbon fiber electrodes (as above), and conventional porous carbon material. Additionally, it is noted that fibrous materials are used in batteries as the fillers and current collectors for the electroactive material, for example, as in U.S. Pat. No. 3,839,090.
In concurrently-filed application Ser. No. 320,250, filed Nov. 12, 1981, there is disclosed an electrode structure usable in the method disclosed herein. Accordingly, the aforementioned concurrent application is hereby incorporated herein by reference.